1. Field of the Invention
The present invention relates to a focusing method of and a focusing control unit for controlling a focal point of recording light upon a master of a data storage medium such as an optical disk. The present invention further relates to a method of and a unit for detecting a deviation of a focal point of recording light upon a master of a data storage medium such as an optical disk. In addition, the present invention relates to a method of and a unit for forming a master of a data storage medium such as an optical disk. The respective methods and unites described above are suitable to where recording light for recording information has a wavelength belonging to the UV region, and therefore, a sensitivity of a detector for detecting the recording light drops down.
2. Conventional Technique
While optical disks have become popular as compact disks (CD) have become prevalent, there still are ongoing hard endeavors to develop optical disks having higher densities. In addition, digital video disks (DVD) having even higher densities than CDs have been proposed and put in an actual use in the recent years. There is an urgent demand for development of an optical disk having an even higher density for future. To this end, it is very important to develop a technique for forming a master of a data storage medium having a high density which allows finer recording.
Conventional methods for forming a master of a data storage medium require, for the purpose of focusing control, to irradiate He—Ne laser light as control light, through an objective lens, upon a glass substrate (disk) which is applied with a photoresist which shows almost no sensitivity to He—Ne laser light which has a wavelength of 633 nm. More specifically, the He—Ne laser light is irradiated through the objective lens, and a focusing-control optical system performs feedback control such that a distance between the objective lens and the glass substrate stays constant in accordance with a change in reflected light from the glass substrate, whereby focusing control is achieved.
Alternatively, laser light having a wavelength of 458 nm or the like to which a photoresist is sensitive is converged and intermittently irradiated, through an objective lens upon a glass substrate, as recording light for recording pits so that a photoresist on the glass substrate is developed and pits corresponding to a modulation signal representing the recording light are formed.
Alternatively, a different focusing control method for feedback control has been proposed and put in an actual practice which uses recording light alone instead of using different control light different from the recording light such as He—Ne laser light.
However, according to the conventional focusing control method above which uses different control light from recording light, since a control position of the objective lens in feedback control, that is, a distance between the objective lens and a recording medium is determined by the focusing-control optical system, independently of a state of the recording light.
With this method, prior to recording other than in an information storage area of a necessary disk, a state of the recording light is confirmed and a focusing control position is accordingly adjusted to an optimal position. During actual recording, feedback control is performed such that a distance between the objective lens and the glass substrate stays constant while fixing the focusing control position. Hence, when the state of the recording light changes during recording and a deviation from the focusing control position is created, the focusing-control optical system can not detect the deviation, and therefore, the deviation can not be corrected.
A change in the state of the recording light during recording is caused by the following, for example. A spread angle of laser light used as the recording light, which is an element forming a unit for forming a master of a data storage medium, changes, or a distance between two lenses changes due to a change in a temperature of a lens holder of a beam expander and the recording light itself consequently changes, for instance. A temperature of an optical unit holder in the focusing-control optical system positioned on a movable optical table changes, thereby causing a positional and an angular changes. The position of an optical axis is accordingly changed, and hence, a target position for focusing control is changed. Due to a change in the recording light itself or a change in a target position for focusing control, the target position for focusing control deviates from an optimal convergence position of the recording light during recording.
Meanwhile, the feedback method using recording light itself solves the problem above regarding the focus position. However, with a currently used recording unit for recording on a master of a data storage medium using UV light, since the intensity of recording light is small, reflected light from a recording medium is weak and a sensitivity of a light detecting apparatus is low, and therefore, it is not easy to obtain a signal which is sufficient for feedback control for focusing control.
Further, after irradiating the recording light upon the recording medium in which a photosensitive material film (photoresist) is applied to a surface of a glass base (glass disk), the photosensitive material film is exposed and developed, and an elliptic pit or a continuous groove is formed in the photosensitive material film of the recording medium. At this stage, the focusing control system performs focusing control, aiming at ensuring that a focal point of the recording light matches with a surface of the photosensitive material film.
However, as focusing control is performed such that the surface of the photosensitive material film is at the position of the focal point, since a film thickness of the photosensitive material film is not a negligible value in the focusing control system, the recording light gradually spreads out until reaching an interface between the glass base and the photosensitive material film after impinging upon the surface of the photosensitive material film. When the photosensitive material film is exposed in this condition, an average diameter of the recording light in the photosensitive material film increases, and therefore, it is difficult to accurately form a fine pit.
Further, even when focusing control is realized in a stable manner, the focusing control system has a deviation (a focusing deviation, as generally known) which is determined by a gain of the feedback control system, and the focusing control position varies within a range of the deviation. Hence, in a condition that focusing control is performed such that the surface of the photosensitive material film is at the position of the focal point, the position of the focal point of the recording light varies in accordance with a focusing deviation and is often located outside the photosensitive material film. This increases an amount of out-of-focus, expands the average diameter of the recording light even larger, and makes it further difficult to accurately form a fine pit.